Variable geometry turbines are well known and generally comprise a turbine chamber within which a turbine wheel is mounted, an annular inlet passageway arranged around the turbine chamber, an inlet chamber arranged around the inlet passageway, and an outlet passageway extending from the turbine chamber, the passageways and chambers communicating such that pressurised gas admitted to the inlet chamber flows through the inlet passageway to the outlet passageway via the turbine chamber. In one common type of variable geometry turbine, one wall of the inlet passageway is defined by a moveable wall member, generally termed “nozzle ring”, the position of which relative to a facing wall of the inlet passageway is adjustable to control the width of the inlet passageway. The inlet passageway width and thus the geometry of the turbine is varied so that as the volume of gas flowing through the turbine decreases the inlet passageway width is also decreased to maintain gas velocity and hence turbine efficiency.
It is also well known to improve turbine efficiency by providing vanes, referred to as nozzle vanes, in the inlet passageway so as to deflect gas flowing through the inlet passageway towards the direction of rotation of the turbine wheel. Nozzle vanes are provided in both fixed and variable geometry turbines. In the latter case the provision of vanes complicates the variable geometry structure, in particular to ensure that the vanes always extend across the full width of the inlet passageway.
U.S. Pat. No. 4,499,732, for instance, describes a variable geometry arrangement in which the vanes are fixed in position but extend through slots in a moveable nozzle ring. Thus, as the nozzle ring moves to control the width of the inlet the vanes will always extend across the full width.
Other variable geometry structures are described for example in U.S. Pat. No. 4,292,807, and British patent specification numbers GB-A-1138941 and GB-A-2044860. These specifications describe various arrangements in which the nozzle vanes extend from a moveable nozzle ring into slots provided on the facing wall of the inlet passageway. This arrangement also ensures that the vanes always extend across the full width of the passageway, even when the passageway is fully open.
Although the provision of nozzle vanes optimises turbine efficiency, the vanes have a disadvantage of reducing the effective area of the turbine inlet so that the maximum gas flow rate through the turbine is less than would be possible if the vanes were not present. U.S. Pat. No. 4,973,223 describes a variable geometry turbine in which the nozzle ring can be “over-opened”, i.e. withdrawn beyond the nominal full width of the inlet passageway, and in doing so retract the vanes at least partially from the inlet passageway. Efficiency of the turbine drops as the vanes are retracted but the increase in maximum flow rate enables a wider range of engine speeds to be matched by the turbine. Although turbine efficiency begins to drop off as the vanes are retracted from the inlet passageway, efficiency may still be greater than that achieved at the low flow range of the turbine. Essentially, therefore, controlled retraction of the vanes enables modification of the turbines characteristic efficiency verses flow curve so that for a given flow range the mean turbine efficiency may be increased by avoiding the need to operate the turbine in the less efficient low flow region.
It is also known to achieve the same effect in a simpler way, by modifying the profile of the nozzle vanes. The present applicant produces a variable geometry turbine in which nozzle vanes extend from a moveable nozzle ring which defines one wall of the turbine inlet passageway into slots defined in an opposing fixed wall of the turbine inlet passageway, and wherein the nozzle ring can be over-opened beyond the full width of the passageway. The nozzle vanes have a cut-out at their radially inner edge and towards the end of the vanes remote from the nozzle ring. This cut-out effectively reduces the height of the nozzle vane over a portion of its width (the height of a nozzle vane being its extent parallel to the axis of the turbine, i.e. the extent to which it extends from the nozzle ring). Thus there is a region towards the end of each vane which has a reduced chord, i.e. its effective width opposing gas flow from the inlet chamber to the turbine chamber. When the nozzle ring is in an open position with the inlet passageway fully opened, the reduced chord portion of the vanes extend through the slots. However, as the nozzle ring is over-opened the reduced chord region is retracted from the inlet passageway side wall so that the total effective vane area extending across the inlet passageway is reduced increasing the swallowing capacity of the turbine. By ensuring that the reduced chord region of the each vane is hidden when the inlet passageway is fully open peak turbine efficiency is not adversely affected.
Although good turbine efficiency is clearly desirable, conventional variable geometry turbine designs can be problematical when the turbine is intended for use with an internal combustion engine having an exhaust gas recirculation (EGR) system. In an EGR system a portion of the exhaust gas taken from the exhaust manifold is reintroduced into the inlet manifold of the engine for further combustion with a view to reducing engine emissions. With modern highly efficient variable geometry turbine designs the boost pressure at the inlet manifold can often exceed the exhaust gas pressure at the exhaust manifold making the reintroduction of exhaust gas to the inlet manifold problematical, for instance, requiring dedicated EGR pumps etc.
It is an object of the present invention to obviate or mitigate the above disadvantage.